1. Field of the Invention
The present invention relates to a synchronous rectifying type of DC—DC converter, a DC—DC converter control circuit constituting such a type of DC—DC converter, a monitor circuit for monitoring an operation of a DC—DC converter, an electronic equipment having a DC—DC converter, and a monitoring method of a DC—DC converter for monitoring an operation of the DC—DC converter.
2. Description of the Related Art
Hitherto, a synchronous rectifying type of DC—DC converter is used in a various type of electronic equipment and apparatus. The synchronous rectifying type of DC—DC converter has a function of step-downing a DC voltage obtained through transformation from a commercial power supply by an AC adapter, and a DC voltage obtained from a battery, for example, to a voltage suitable for an operation of an internal circuit. The synchronous rectifying type of DC—DC converter has the advantages of high efficiency and low loss.
FIG. 5 is a block diagram of a synchronous rectifying type of DC—DC converter.
Between an input terminal 100a and an output terminal 100b of a synchronous rectifying type of DC—DC converter 100, a first switch or FET 110 and an inductor 120 are disposed. Between the connecting point of the first switch (FET 110) with the inductor 120 and the ground, a second switch or FET 130 is connected.
FET is a field effect transistor. In a synchronous rectifying type of DC—DC converter, it often happens that as the first switch and the second switch, FET is used. However, any one is acceptable, which is a switch, and it doesn't matter what kind of transistor and switch are concerned with.
It happens that the first switch or FET 110 is referred to as a main switch, a main transistor, a main switch element, a main side switch, a main side FET, a high side switch, or a high side FET.
It happens that the second switch or FET 130 is referred to as a synchronous rectifying switch, a synchronous rectifying transistor, a synchronous rectifying switch element, a synchronous rectifying side switch, a synchronous rectifying side FET, a low side switch, or a low side FET.
Hereinafter, with respect to the “Description of the Related Art”, the above-mentioned switch will be explained using the term of FET which is used typically in the synchronous rectifying type of DC—DC converter.
In the synchronous rectifying type of DC—DC converter 100 shown in FIG. 5, a diode 140 for a flywheel, which is operative when a first FET 110 and a second FET 130 are simultaneously turned off, is connected in parallel with the second FET 130. The DC—DC converter 100 is provided with a control circuit 150 for controlling the first FET 110 and the second FET 130 so that they are alternately turned on. Capacitors C1 and C2, which are connected to the input terminal 100a and the output terminal 100b, respectively, are provided for a stabilization of voltages. The input terminal 100a receives an electric power of a predetermined DC voltage VIN. The control circuit 150 controls the first FET 110 and the second FET 130 so that they are alternately turned on. As a result, there is generated an electric power of a DC voltage VOUT lower than the DC voltage VIN inputted from the input terminal 100a. The electric power thus generated is outputted from the output terminal 100b. The control circuit 150 is typically made of LSI.
FIG. 6 is an illustration showing time variations of turn-on and turn-off of the first FET and the second FET.
Part (A) of FIG. 6 shows turn-on and turn-off of the first FET 110. Part (B) of FIG. 6 shows turn-on and turn-off of the second FET 130. The control circuit 150 controls the first FET 110 and the second FET 130 so that they are alternately turned on. As shown in FIG. 6, there is provided a period of time in which the first FET 110 and the second FET 130 are simultaneously turned off. The reason why this is to do so is that it is prevented that the first FET 110 and the second FET 130 are simultaneously turned on. When the first FET 110 and the second FET 130 are simultaneously turned on, as seen from FIG. 5, the input terminal 100a is grounded through the first FET 110 turned on and the second FET 130 tuned on, so that a large surge current conducts through the first FET 110 and the second FET 130. This brings about a possibility of an occurrence of an erroneous operation on an electronic apparatus being operated by an output of the DC—DC converter owing to lowering of an input voltage of the electronic apparatus. Further, when the surge current exceeds an allowable current of the FET, it involves danger such as smoking and ignition. This brings about a deterioration of reliability of the apparatus.
The diode 140 shown in FIG. 5 operates instead of the second FET 130 in timing that the first FET 110 and the second FET 130 are simultaneously turned on, and has a function of transmitting an electric power to the output terminal 100b side. The diode 140 is larger in a potential drop as compared with the FET, and thus the conversion efficiency is decreased. Accordingly, it is preferable that the period of time that the first FET 110 and the second FET 130 are simultaneously turned on is short as much as possible as far as the surge current is prevented.
As mentioned above, the synchronous rectifying type of DC—DC converter as shown in FIG. 5 has the advantages of high efficiency and low loss. On the other hand, such a DC—DC converter is associated with a problem that a performance of the converter is affected by a performance of the FET. For example, in the event that a circuit, which is large in load, is driven by an output of the DC—DC converter, an FET, which is large in a gate capacity, is used. However, if a driving ability of the control circuit shown in FIG. 5 for the FET is short, before one of the FETs turns off completely, another FET will turn on. This brings about a possibility of an occurrence of a surge current referred to as a short through.
FIG. 7 is an illustration showing time variations of turn-on and turn-off of the first FET and the second FET in the situation as mentioned above.
As seen from FIG. 7, if a driving ability of the control circuit as compared with a gate capacity of the FET is short, it takes a time for translation from the turn-on state to the turn-off state, and before a gate voltage of one of the FETs is lowered to a threshold at which the one FET is turned off, another FET is turned on, and thus there will be generated a term Δt in which both the FETs offer the turn-on state.
The output voltage of the DC—DC converter as shown in FIG. 5 is determined by a duty ratio (a rate of a time on the turn-on state per a period) of the first FET 100. Thus, when the DC—DC converter receives a voltage which exceeds a rated current, the control circuit 150 serves to extremely lower a duty ratio of the first FET 100 in order to obtain an output of a constant voltage. However, the control circuit 150 is also associated with the minimum turn-on time that the control circuit 150 cannot operate normally when a pulse width goes down to a certain value or less. And thus when the pulse width goes down to the certain value or less, the control circuit 150 will erroneously operate, and as a result, it is considered that the output voltage is unstable and the surge current is generated.
In view of the foregoing, according to the synchronous rectifying type of DC—DC converter, there are determined limits of the input voltage and the output voltage, and the maximum output current, and as shown in FIG. 6, there is provided a time in which both the FETs maintain the turn-off state, to prevent an occurrence of the surge current and the like.
However, for example, in the event that an LSI constituting a control circuit is adopted, there is a possibility that the control circuit is not used in accordance with the specification and a large gate capacity of FET is used so that a large current is derived from the DC—DC converter. Alternatively, even if the DC—DC converter or an electronic apparatus incorporating therein the DC—DC converter is used completely in accordance with the specification in the fabrication step of those apparatuses, in some user of the electronic apparatus incorporating therein the DC—DC converter, there is a possibility that an AC adapter of an electronic apparatus other than an AC adapter for example which will be described latter is connected, and as a result, a voltage exceeding a rated input voltage is applied to the DC—DC converter.